Mechanism of transposition of bacteriophage Mu: polarity of the strand transfer reaction at the initiation of transposition.

The distribution of newly synthesized DNA strands in the transposition products of bacteriophage Mu made in an in vitro system has been analyzed. The results support a model in which all Mu transpositions are initiated by a pair of strand transfer reactions that attach the 3' ends of Mu DNA to 5' protruding staggered ends of the target DNA. Joining of these ends produces a pair of structures similar to replication forks at the ends of the Mu DNA. Successful initiation of replication at either one or both ends, followed by a round of semiconservative replication, results in formation of a cointegrate structure. When the intermediate structure fails to replicate, breakage of the junctions between the Mu sequence and the vector sequence derived from the donor molecule can lead to a simple insert with a pair of gaps at the 5' ends of the Mu DNA. Evidence for a gap repair process that completes the simple insertion process has been obtained.